Automated guided vehicles are known for various material handling applications which generally involve moving loads around a facility such as a warehouse. To this end, an automated guided vehicle (AGV) comprises a vehicle chassis having a plurality of wheels which allow the vehicle to traverse a floor of the warehouse. The AGV also comprises a mast and a lift carriage, the lift carriage being vertically movable along the mast to allow the AGV to deposit and retrieve loads from various heights. The AGV is thus able to pick up, move and deposit loads at various locations within the warehouse.
To achieve vertical mobility of the lift carriage along the mast, the AGV further comprises a power drive assembly formed of a vertically oriented ball screw matingly engaged with a lift nut. The ball screw is coupled to an electric motor which drives the ball screw. The lift nut is mounted (though not necessarily directly) to the lift carriage. As the ball screw rotates, it imparts a vertical force on the lift nut and thereby on the lift carriage. The lift carriage further comprises rollers which are disposed in vertically oriented brackets of the mast and which guide the vertical movement of the lift carriage. The lift carriage is thus raised or lowered (depending on the direction of rotation of the ball screw) along the mast as the ball screw rotates.
An important condition for the proper rotation of the ball screw with respect to the lift nut is that the ball screw and the lift nut must be in perfect alignment. To the extent that the ball screw and lift nut are misaligned, horizontal forces and angular moments between the ball screw and the lift nut contribute to a premature wearing out of the ball screw. It has been found that a misalignment of only a few thousandths of an inch can significantly decrease the life of the power drive assembly.
Generally speaking, misalignment of the lift nut with respect to the ball screw is caused by a misalignment of two or more of the remaining components of the mast, power drive assembly and lift carriage. Accordingly, one way to ensure near-perfect alignment is to use highly precise manufacturing tolerances when specifying the dimensions of the respective components of the mast, power drive assembly, and lift carriage. However, this approach is unsatisfactory because highly precise manufacturing tolerances make the respective components expensive to manufacture.
Thus, what is needed is an alternative way to avoid the application of angular moments and translational forces between the ball screw and the lift nut and thereby increase the life of the ball screw assembly.